The present invention relates to a battery pack for power tool which includes a rechargeable battery which is repeatedly rechargeable, and a power tool to which the battery pack for power tool is attached and which is operated by receiving power supply from the battery pack for power tool.
In a battery pack for power tool using a battery which includes a lithium-ion rechargeable battery (hereinafter also referred to as “a battery pack”), there is generally provided a monitoring circuit. The monitoring circuit is operated using the battery as a power source. The monitoring circuit is provided because it is necessary to always monitor a status of the battery while being charged or while discharging to the power tool which is an object to be supplied with power from the battery. Items to be monitored by the monitoring circuit includes, for example, a voltage of each battery cell composing the battery, a temperature of each battery cell (or a temperature of the entire battery), a charge/discharge current to/from the battery, and the like. A voltage of the entire battery based on a total sum of the voltage of each battery cell may also be one of the items to be monitored.
When the monitoring circuit, which is always in operation, is provided in the battery pack as above, it is inevitable that electric power of the battery is always consumed by the monitoring circuit although in a small amount. Consequently, even if the battery is not supplying electric power to the power tool, a remaining battery capacity gradually decreases to zero in a comparatively short period of time.
Meanwhile, in view of an object of the monitoring circuit, when the battery is neither being charged nor discharging to the power tool, that is, when not in use, the battery is generally in a stable state and is not necessarily required to be monitored. Therefore, a technique is conventionally known in which operations of the monitoring circuit are stopped by switching the battery pack to a sleep mode when the battery is not in use.
A timing to switch the battery pack to the sleep mode has been suggested. One example thereof is disclosed in Unexamined Japanese Patent Publication No. 2003-264008. In that publication, it is suggested that when a load is disconnected from an accumulator battery, immediately or subsequently thereafter, a supply of a load-dispatching energy to a circuit inside the accumulator battery is stopped. Another example is disclosed in Unexamined Japanese Patent Publication No. 2006-280043. In that publication, it is suggested that, once operations of a trigger switch in a power tool are released, power supply to a control unit inside a battery pack is stopped after a lapse of a predetermined period of time.
In the above-described method, while the battery is discharging (e.g., when the power tool is in use), a battery voltage is decreased and a battery temperature is increased by the discharge current and an internal resistance inside the battery. Then, when the discharge is finished, the battery voltage is increased to return to an original open voltage value, and the battery temperature drops to return close to ambient temperature, whereby the battery goes into a stable state.
Due to properties of the battery, however, the decreased battery voltage and the increased battery temperature as a result of the discharge do not recover immediately after the discharge is finished, but require a certain period of time to return to an original state. In other words, the battery gradually returns from an immediate state after the discharge and, conversely, the battery cannot be considered to be chemically stable while gradually returning to its original state. And while the battery is in such an unstable state, an abnormality may occur in the battery due to a certain cause.
Examples of such an abnormality in the battery occurring in the unstable state immediately after the discharge include a slight short circuit in battery cells composing the battery. The slight short circuit is a phenomenon in which interelectrode portion inside the battery cell is short-circuited due to a certain cause.
When the interelectrode portion inside the battery cell is short-circuited to cause the slight short circuit, the short-circuited portion is promptly fused by the short-circuit current (i.e., a momentary short circuit). The battery in which the slight short circuit has occurred appears to be normal for the moment. However, even such a momentary short circuit remains an abnormality which has occurred inside the battery cell. Therefore, it is necessary to provide a process which can detect the occurrence of the slight short circuit and make the battery unusable after the detection.
However, if the operation of the monitoring circuit is stopped by switching the battery pack to the sleep mode immediately after the discharge is finished in order to reduce electric power consumption of the battery, the abnormality such as the above slight short circuit cannot be detected. That is because the monitoring circuit is not in operation for some period of time after the discharge is finished, when the unstable state continues in which the abnormality such as the slight short circuit is highly likely to occur as described above.
Instead of switching the battery pack to the sleep mode immediately after the discharge, it is possible for the monitoring circuit to be in operation at least until the battery goes into a stable state if the battery pack is switched to the sleep mode after a predetermined period of time as described in the above patent document.
However, it is extremely difficult to accurately determine the time period required until the battery goes into a stable state after the discharge because such a time period is greatly influenced by the current and temperature during the discharge, a level of deterioration of the battery cell, and the like. Consequently, the time period cannot help but be set comparatively long, so that the monitoring circuit is steadily in operation until the battery goes into a stable state, even in a case where it takes the longest time for the battery to go into a stable state.
When the time period soon after the discharge until the battery pack is switched to the sleep mode is predetermined to a relatively long time, it is highly assumed (or rather it is often the case) that the battery goes into a stable state before the predetermined time elapses. Therefore, it is difficult to effectively reduce the electric power consumption of the battery.
Such a problem may occur in various battery packs which include a monitoring circuit for monitoring a state of a battery, as well as in the battery pack including lithium-ion rechargeable batteries.